Lots of helper functions, track days of construction left (for enqueued areas).
Data for how long buildings take to build.
In do_turn(), progress work on areas.  When enqueuing a new area to build, set it as a queued area (i.e. set its construction_left).
Date::standardize().
Game::advance_time() may be passed a pointer to a City; if so, it calls City::do_turn() an appropriate number of times.
Keybinding for building a new area is now A, not B (B conflicted with vikeys, so sad).